1. Field of the Invention
The present invention relates to a fuel cell separator unit and a fuel cell stack.
2. Description of the Related Art
Conventionally, fuel cells generate electricity very efficiently and do not discharge harmful substances, so they have been put to use as industrial and household electrical generation units and as power sources for man-made satellites, spacecraft, and the like. However, in recent years their development has been progressing as power sources for use in vehicles such as passenger cars, buses, trucks, passenger carts, freight carts, and the like. Fuel cells of an aqueous alkaline solution type (AFC), a phosphoric acid type (PAFC), a molten carbonate type (MCFC), a solid oxide type (SOFC), a direct methanol type (DMFC), and the like may be used, but a solid polymer type fuel cell (PEMFC), in which pure hydrogen serves as the fuel gas, is being actively used, because the volume and weight of the system per unit output is low.
In the PEMFC, a membrane electrode assembly (MEA) is used in which a solid polymer electrolyte membrane is sandwiched between two gas diffusion electrodes to form a single unit. One of the gas diffusion electrodes serves as a fuel electrode (anode). When hydrogen gas is supplied as a fuel to the surface of the fuel electrode, the hydrogen dissociates into hydrogen ions (protons) and electrons, and the hydrogen ions move through the solid polymer electrolyte membrane. The other gas diffusion electrode serves as an oxygen electrode (cathode). When air is supplied as an oxidizing agent to the surface of the oxygen electrode, the oxygen in the air bonds with the hydrogen ions and electrons to form water. Electromotive force is generated by this electrochemical reaction.
The solid polymer electrolyte membrane fuel cell has a layered structure in which a separator, which forms a flow passage for reaction gases such as the hydrogen gas and the air, is disposed on the outside of the MEA. The separator prevents the reaction gases from permeating the MEA, which is adjacent to the separator in the layering direction, and also serves as a power collector that directs the electric current generated by the MEA to the outside. A fuel cell stack is built by stacking pluralities of MEAs and separators. Also, a technology has been proposed (for example, in Japanese Patent Application Publication No. JP-A-2005-285685) that uses a metal sheet with a mesh of expanded metal or the like as a collector for the separator, in order to supply the reaction gases to the MEA, reduce power collection resistance, maintain the solid polymer electrolyte membrane in a moist state, and the like.
However, in the conventional separator, the metal sheet with the mesh that is used is a folded sheet with a rectangular cross section. This not only increases the thermal resistance in the layering direction, resulting in inadequate cooling of the electrodes, but also raises concern that the water will obstruct rib-shaped portions, interfering with the supply of the gases. Moreover, the metal sheet with the mesh is itself easily deformed, which creates problems in the distribution of surface pressure.